The present invention relates to an apparatus for protecting devices for measuring some aspect of hot environments against heat damage. The present invention in its preferred embodiment is used to protect oxygen concentration probes from heat damage while they are within exhaust system of steam generators.
In the typical steam generator many feet of piping carries water through the steam generator. The water enters the piping and exits as steam after being exposed to the heat generated within the steam generator. It is highly advantageous for energy conservation purposes to keep the air and fuel mixtures under control to optimize the mix for maximum efficiency in combustion. The steam generator typically includes devices which control the air intake and the fuel intake to the combustion chamber of the steam generator. These devices typically include a digital computer for automatically adjusting the air and fuel mix for optimum efficiency. However, in order for the computer to know the optimum air/fuel mix at any particular time, the computer must be continuously fed information regarding the oxygen concentration in the combustion exhaust.
In order to know the true oxygen concentration of the combustion exhaust without possible contamination from the outside air, measurements of this concentration must be made sufficiently close to the combustion chamber to assure that outside air could not reach the measuring point and result in false measurements. Measurements within the combustion chamber would assure that no outside air would lead to false measurements because of the combustion chamber area is air tight. However, the combustion chamber has operating temperatures of approximately 2500 degrees Fahrenheit. Very few oxygen concentration probes can withstand such temperatures, and, in any event, the ones which can are extremely expensive and become fouled easily because of the concentration of dusty combustion products and gases.
At the flue of the steam generators there is the greatest possibility that outside air would contaminate any measurements of oxygen concentration although this position offers the lowest operating temperature (approximately 400 degrees Fahrenheit). Thus, the flue is completely unsuitable because of the potential for outside air contamination. However, in a median position within the combustion exhaust system prior art measurement systems have found some success in measuring oxygen concentrations without significant outside air contamination. Nevertheless, temperatures ranging from 1400 to 1600 degrees Fahrenheit exist at this median position. Temperatures of 1400 to 1600 degrees Fahrenheit are well over the maximum temperatures that may be withstood by the majority of oxygen concentration probes. Even oxygen concentration probes of the highest temperature ratings in this median position are subject to many other problems that will be discussed below.
In order to reduce the temperature of the combustion gases and products to a level which may be withstood by most oxygen concentration probes, the prior art uses a pipe which circulates combustion gases and products outside the steam generator near the median position. The pipe allows the combustion gases and products to cool significantly before they reach the oxygen concentration probe installed into pipe.
There are many inherent disadvantages of this prior art system for facilitating oxygen concentration measurements. First and most obvious is that by allowing some of the combustion gases and products to cool in the circulation pipe, the steam generator is allowing precious heat energy to be lost. This lost heat energy leads to a less efficient steam generator. Second, the safety of maintenance personnel is greatly compromised by being exposed to hot piping surfaces and to potential rupture or perforation of the pipe and its consequent sudden exposure to high temperature gases and toxic fumes.
Third, the cooling of the combustion gases via the circulation pipe tends to cause highly corrosive condensation products (such as concentrated sulfuric acid) to accumulate in the pipe and around the probe. These products reduce the life of the pipe and probe and add to the danger of injury to maintenance personnel as described above. Fourth, the expense to install and maintain the circulation pipe and the probe is considerable.
The prior art has also attempted to install the probe within the steam generator without the aid of cooling effect of the circulation pipe by placing the probe within a water jacket (unshown) of the type described in U.S. Pat. No. 3,643,508. The description of this water jacket of U.S. Pat. No. 3,643,508 is incorporated herein by reference in its entirety. These water jackets have even more disadvantages than the above described cooling system comprising the circulation pipe. First, the cooling liquid must be pressurized to flow through the jacket. This necessitates expensive and complex pumping apparatus. Second, pressurized cooling liquid is corrosive in nature which means that the system requires continuous maintenance. Third, the temperature of the cooling liquid must be monitored and controlled to avoid over cooling or under cooling. Under cooling will inevitably lead to the boiling away of the cooling liquid and the destruction of the probe. Over cooling can affect the accuracy of the probe and will cause the formation of excessive condensation products around the jacket which are typically highly corrosive. Fourth, the jacket necessarily causes the formation of condensation products which are typically highly corrosive and injurious to the jacket and its installation within the steam generator. The reason for causing the formation of condensation products is that the probe will be cooled below the dew point of the hot gases. Fifth, the jackets are costly to install and maintain.
Other prior art inventions are known which relate generally to protection of probes within hot environments. These are found in U.S. Pat. Nos. 4,668,477; 4,569,228; and 2,122,697. However, no known prior art invention uses convection cooling as does the present invention.
The present invention was created in response to these obvious disadvantages of the prior art systems. As will be shown in the following description of the preferred embodiment, the present invention has successfully eliminated or greatly diminished all of the above described disadvantages.